Environmental concerns have motivated the implementation of emission requirements for internal combustion engines and other combustion systems throughout much of the world. Catalytic converters have been used to eliminate many of the pollutants present in exhaust gas; however, a filter is often required to remove particulate matter, such as, for example, ash and soot. Wall-flow particulate filters, for example, are often used in engine systems to remove particulates from the exhaust gas. Such particulate filters may be made of a honeycomb-like substrate with parallel flow channels or cells separated by internal porous walls. Inlet and outlet ends of the flow channels may be selectively plugged, such as, for example, in a checkerboard pattern, so that exhaust gas, once inside the substrate, is forced to pass through the internal porous walls, whereby the porous walls retain a portion of the particulates in the exhaust gas.
In this manner, wall-flow particulate filters have been found to be effective in removing particulates from exhaust gas. However, the pressure drop across the wall-flow particulate filter increases as the amount of particulates trapped in the porous walls increases. The increasing pressure drop results in a gradual rise in back pressure against the engine, and a corresponding decrease in the performance of the engine. Accordingly, soot is commonly oxidized and removed in a controlled regeneration process before excessive levels have accumulated.
The ability to measure or estimate the amount of particulate, such as, for example, soot accumulated in a particulate filter is valuable as it helps to determine the regeneration schedule for the filter. Optimizing a filter's regeneration frequency, for example, can reduce the negative impacts of regeneration (e.g. increased emissions and fuel consumption) from too frequent regeneration, and protect the filter from over-exposure and possible failure due to the heightened energy release caused by excessive particulate loading from too infrequent regeneration. Accurately estimating the particulate load level (e.g., soot load level) in a particulate filter may thus facilitate determining when to initiate a timely and controlled regeneration event.
Conventional methodologies for estimating soot load in a particulate filter include both pressure drop based techniques and mass balance based techniques. Pressure drop based systems and methods have been shown to accurately estimate soot load in a particulate filter even under very dynamic operating conditions. Under certain engine operating conditions, however, notably during long idle times, during active regeneration and during periods of filter ash conditioning, pressure drop based methods can also have some limitations, thereby providing somewhat less accurate soot load estimates during such periods.
On the other hand, although having somewhat limited accuracy under dynamic operating conditions and regions of fast transients, mass balance based techniques for estimating soot load can generally be relatively accurate during steady state conditions, including, for example, during long engine idle times and active regeneration. Furthermore, due to ash accumulation effects, mass balanced based estimation approaches are generally more reliable during periods of filter ash conditioning, for example, during the first few thousand miles of filter exposure to engine operation. Ash conditioning refers to a period of ash accumulation on a filter's wall surfaces. Prior to the formation of an ash layer, particulate matter is able to penetrate the filter wall, a filtration mode commonly referred to as deep-bed filtration. Once an ash layer is formed, however, particulate matter is unable to penetrate the filter wall and is primarily captured on the surface of the filter wall, a filtration mode commonly referred to as cake-bed filtration.
It may be desirable, therefore, to provide an approach for estimating soot load that provides relatively high levels of estimation accuracy throughout all periods of engine operation, for example, during both dynamic operating conditions and steady state conditions, long idle times and active regeneration, and during periods of filter ash conditioning. It also may be desirable to provide an approach for estimating particulate load that is relatively simple to implement, using the instrumentation and sensors already available as part of an engine's after-treatment system.